Research has been progressing into silicon carbide SiC as a semiconductor device for high performance power devices or as a substrate for devices having high voltage, low-loss and which are operable at high temperatures in view of silicon carbide SiC's excellent physical properties of wide bandgap, a high reverse blocking voltage and high thermal conductivity.
To produce a SiC semiconductor device, a SiC single crystal having a doping concentration-controllable SiC single crystal film formed on a SiC single crystal substrate is often used. For this reason, forming the SiC single crystal film on the SiC single crystal substrate by epitaxial growth using chemical vapor deposition (CVD) is performed. In CVD, a source gas containing gaseous carbon and silicon atoms is supplied onto a SiC single crystal substrate under high temperature and normal or reduced pressure to form a SiC single crystal film on the SiC single crystal substrate.
Currently, the SiC single crystal substrate used for forming the SiC single crystal film is a commercially available SiC single crystal substrate formed by sublimation. A SiC single crystal substrate 100 such as this comprises micropipes (hollow-core defects) 101 in a high density of 101 to 103/cm2, as illustrated in FIG. 13A. When forming the SiC single crystal substrate 100 by sublimation, screw dislocations 102, as illustrated in FIG. 13A, are generated in a huge number. Among them, the screw dislocations 104 in FIG. 13B having a small Burghers vector, which is described as the one unit of step gap, do not become hollow-cores, while the screw dislocations 104 having a large Burghers vector 105 become hollow-cores to reduce their core energy, whereby micropipes 101 are formed. In other words, micropipes 101 are large screw dislocations 104.
As illustrated in FIGS. 9 to 12, during epitaxial growth by CVD these micropipes (hollow-core defects) can propagate without being closed. Even for bulk growth by sublimation, because micropipes propagate from the seed crystal to the bulk crystal, in newly formed crystals during bulk growth, micropipes replicate in the same density as in the substrate and the seed crystal. The micropipes in FIGS. 9 to 12 are described as having three unit of steps of the Burghers vector (one unit of the step is the height of one unit of SiC lattice).
A semiconductor device containing micropipes 101, as illustrated in FIG. 8, does not satisfy the necessary characteristics of a semiconductor device in terms of breakdown voltage, so that a semiconductor device that contains even one micropipe 101 is treated as a defective device. For this reason, the ratio of non-defective devices among all semiconductor devices (yield) is strongly influenced by the micropipe density of the SiC single crystal substrate 100. When the surface area of the semiconductor device is enlarged, the probability that micropipes 101 will be contained within the semiconductor device area increases, meaning that yield drops, which is an obstacle to enlarge the device area. When a semiconductor device is fabricated using an epilayer grown on the SiC single crystal substrate 100 which contains a large number of micropipes 101, a large number of small surface area electrodes have to be formed on the SiC crystal film 103 in order to obtain a semiconductor device which does not contain micropipes 101, which is very troublesome. Accordingly, a low-defect, if possible a zero-defect, SiC single crystal film is desired.
To obtain a doping concentration-controllable SiC single crystal film, a technique is employed for controlling the crystal dopant concentration which lowers the amount of impurities that are unintentionally mixed therein as far as possible, then from that state flows a predetermined flow amount of, for example, nitrogen gas or the like for an n-type. For a p-type, a gas containing aluminum or the like is employed. Therefore, the fabrication of a film having a low amount of impurities unintentionally mixed therein (high purity SiC film) is required.
To fabricate a semiconductor device using the formed SiC single crystal and carry out secondary crystal growth, the surface shape (morphology) of the SiC single crystal film is preferably as smooth as possible.
It is an object of the present invention to provide a SiC crystal, and the fabrication process thereof, having a reduced number of hollow-core defects, called micropipes, that have propagated from a SiC single crystal substrate, even if a SiC single crystal substrate having hollow-core defects is used.
It is another object of the present invention to provide a SiC single crystal film and a SiC single crystal substrate, which have a reduced number of hollow-core defects that have propagated from the substrate and that can be doped with a predetermined concentration of dopant.
It is a further object of the present invention to provide a SiC semiconductor device and an electronic device.
It is another object of the present invention to provide a fabrication process of SiC bulk crystal having a reduced number of hollow-core defects.